1. Field of the Invention
The present invention relates to a novel process for separation of naphthalenedisulfonic acids. More particularly, the invention relates to a process for separating 1,6-naphthalenedisulfonic acid, 2,6-naphthalenedisulfonic acid or 2,7-naphthalenedisulfonic acid selectively from reaction mixtures obtained by disulfonation of naphthalene according to known disulfonation reactions which favor the formation of a particular isomer.
2. Description of the Prior Art
As is well known in the art, naphthalenedisulfonic acids are important and valuable intermediates for use in the manufacture of dyes, organic pigments, medicines and agricultural chemicals.
Generally, naphthalenedisulfonic acids can easily be prepared by sulfonation of naphthalene with a sulfonating agent such as sulfuric acid, fuming sulfuric acid or chlorosulfonic acid. When naphthalene is sulfonated at 80.degree. C., the product is chiefly 1-naphthalene sulfonic acid. When it is sulfonated at 160.degree. C. or higher, the yield is chiefly 2-naphthalene sulfonic acid. When the 1-naphthalene sulfonic acid is heated in sulfuric acid at 160.degree. C., it is largely converted to the 2-isomer. The alpha-sulfonation occurs more rapidly because of the more stable intermediate carbonium ion. Sulfonation at the beta position occurs more slowly but, once formed, the beta-sulfonic acid tends to resist disulfonation.
At low temperatures, disulfonation is slow. Therefore, the isolated product is the product which is formed faster, i.e., the alpha-naphthalene sulfonic acid. At higher temperatures, the disulfonation becomes a more important factor, equilibrium is more readily established and the isolated product is the more stable beta-naphthalene sulfonic acid. At low temperatures, the controlling factor is the rate of reaction whereas, at high temperatures, the controlling factor becomes the position of equilibrium.
By controlling the various reaction parameters, the applicants are able to produce a disulfonated naphthalene which contains a high concentration of a particular isomer of the disulfonated naphthalene. However, the disulfonation product ordinarily contains various isomers such as 1,6-naphthalenedisulfonic acid, 2,6-naphthalenedisulfonic acid and 2,7-naphthalenedisulfonic acid, and it is very difficult to separate a specific naphthalenedisulfonic acid at a high purity from this reaction product mixture. Thus, only an extremely limited number of processes have been proposed for the separation and/or purification of naphthalenedisulfonic acids.
For example, as the known process, there can only be mentioned a process in which sodium 2,6-naphthalenedisulfonate is separated by utilizing the solubility difference among salts of naphthalenedisulfonic acids and a process in which aniline is added to a mixture of naphthalenedisulfonic acids to form aniline salts of naphthalenedisulfonic acids and separation is carried out by utilizing the solubility difference among the aniline salts.
A certain separation effect can be attained in these known processes, but they still involve defects or disadvantages to be eliminated.
More specifically, in the former process, since the solubility difference among isomers of naphthalenedisulfonic acid sodium salts is not conspicuous at room temperature, the solubility difference at elevated temperatures should be utilized. Accordingly, an apparatus capable of resisting high temperatures should be employed and, when an operation is conducted at high temperatures, it always involves risks.
In the latter process, since aniline, one of the organic solvents, is used for separation, the steps of recovering and purifying aniline are necessary and an increase in the costs of products is inevitably brought about. Therefore, this process is not a preferred one from an industrial viewpoint.
As will be apparent from the foregoing illustration, the known processes for separating specific naphthalenedisulfonic acids from disulfonation reaction mixtures have always involved one problem or another.